A steering power-assistance arrangement generally includes an assistance pump which is usually in the form of a vane-type pump, as can be seen from European Pat. No. 68 035. In the installed condition of the pump, it is connected to a hydraulic steering arrangement which includes a steering valve, the position of which depends on the position of the steering wheel which forms part of the steering system of the motor vehicle whose steering is to be power-assisted. The steering valve is supplied with a controlled working or output flow which is controlled by a flow control valve arranged in the pump housing. In order to provide a compact construction, a pressure relief valve for limiting the pressure in the system, in the form of a pilot control valve, is combined with the flow control valve as the main valve component, whereby the relieved flow of hydraulic fluid is returned into the feed system of the pump, over a short distance.
Now, when parking a motor vehicle with such a power assistance arrangement, it may happen that the steering system is moved into one of its limit positions, in other words the steering wheel of the vehicle is turned until it reaches a limit position in which it is prevented from further rotation in that direction, whereby the controlled output flow is shut off. As a result of that, the pressure in the steering assistance arrangement rises substantially so that the pressure relief valve then responds. In such a situation, a large amount of power is converted at the combined valve arrangement, and that gives rise to a corresponding increase in the temperature of the hydraulic oil in the system. Where, in such circumstances, the hydraulic fluid is circulated within the pump between the feed system and the discharge system of the arrangement, the hydraulic fluid does not experience any cooling effect by flowing through the other hydraulic lines between the pump and the steering assembly of the vehicle, so that the hydraulic oil temperature in the pump can reach a level of more than around 250.degree. C., within a few seconds. If the power-assistance pump is designed with a housing of aluminium or an aluminium alloy, in order to provide a saving in weight, the substantial thermal expansion of the material of the housing of the pump gives rise to adverse affects in respect thereof; for example where the pump is a vane-type pump comprising a housing made up of a main housing portion and a flange-like cover portion which is fitted on to the main housing portion, then thermal expansion of the housing structure can result in the middle part of the cover portion of the housing deflecting towards the rotor which is rotatable within the housing, so that the cover portion rubs against the rotor and can ultimately cause it to jam, with the result that the drive shaft of the system is damaged or possibly broken.
In one form of vane-type pump for power steering systems, as disclosed in European Pat. No. 14 836, the pump comprises a housing formed by a cup-shaped housing portion and a bowl-like cover portion which together define the internal chamber of the pump. Disposed in the internal chamber of the pump is an insert assembly comprising a pressure plate, a wear plate, a cam ring and a rotor, with a spring being provided to hold the components of the insert assembly together. In a practical case (Vickers VT 50-pump) the above-mentioned pressure plate is 13 mm in thickness while the highest pressure involved in the pump is around 100 bars. The side plates comprising the above-mentioned pressure plate and wear plate are made of sintered steel and do not have any coating of bearing metal thereon, so that the rotor, as it rotates, bears directly against the pressure plate and the wear plate. That system includes a combined flow control and pressure relief valve which extends parallel to the axis of the machine and which, in the event of the pressure relief function becoming operative, gives rise to a short-circuit between the outlet ports and the inlet ports of each displacement region of the pump, thus resulting in a substantial increase in temperature. Thermal expansion which may occur can be readily accommodated by the insert assembly being displaced against the force of the above-mentioned spring which holds the components thereof in the assembled condition. The inlet ports of that system are arranged between the cam ring and the pressure plate of the pump, in other words, apertures in the pressure plate serve only as outlet ports. There is therefore no need for the inlet ports to be sealed off in the region behind the pressure plate.
Another vane-type pump, as disclosed in German Patent specification No. 2 735 663, comprises a housing formed by an annular housing portion and a cover portion which co-operate to define the internal chamber of the pump. Side plates which are provided with bearing metal on the operative surfaces thereof are clamped between the housing portion and the cover portion, and enclose the rotatable rotor with its vanes. The above-mentioned side plates comprise steel and, in the layer of bearing metal applied thereto, have harder portions of an island-like configuration, with which the plates are supported against the parts of the housing. The side plates are immovable in the axial direction of the machine, in contrast to the pressure plate which, when the pump is in the form of a steering power-assistance pump (as in European Pat. No. 14 836), has a certain amount of axial displaceability within the housing of the pump, in order thereby to be urged towards the rotor by the pressure generated within the pump. In addition, the system disclosed in German Pat. No. 2 735 663 does not have a combined flow control and pressure relief valve within the housing, so that hydraulic fluid under high pressure is not relieved into the feed or intake system within the pump, over a short flow distance. That arrangement therefore does not involve the risk of the pump rapidly overheating when the pressure relief function comes into operation, because the system is protected by a pressure relief valve which is disposed at a more remote position. The remote positioning of the pressure relief valve therefore affords a possibility of the hydraulic oil being cooled as it flows through conduits in the system. In existing pumps of that kind, for example the VQ-pumps from Vickers Inc., Troy, United States of America, the housing comprises cast iron so that the problems of excessive thermal expansion do not arise.
A further example of side plates which are clamped in position in the pump housing, with bearing metal on the operative surfaces thereof, is to be found in European Pat. No. 68 354 B1. That arrangement also does not have a combined flow control and pressure relief valve within the housing of the pump, and the pump cannot be used for steering power assistance purposes.